A set of nuclear-encoded proteins that bind with high specificity and affinity to the 5'-untranslated region (UTR) of the chloroplast- encoded psbA mRNA have been identified in the unicellular green alga Chlamydomonas reinhardtii. Binding of these proteins to 5'-UTR is required for translation of the psbA mRNA and is regulated through elevated ADP levels and redox potential; two physiological events that are linked to photosynthesis. The cDNAs representing three of these translational activators have been isolated. Sequence analysis of two of the cDNAs has revealed that the corresponding proteins represent a poly A binding protein and a protein disulfide isomerase. A third protein at this time has no assigned function. We have produced antisera against these proteins, and have isolated two of the genomic clones: a screen of a genomic library should allow isolation of the third clone. We plan to characterize the primary signal and other components in the signal transduction pathway that regulate each of the nuclear-encoded psbA binding protein genes in order to understand how their expression influences translation of the psbA mRNA. These studies represent the first to address the characterization of cloned translational activators in plants, and will provide insight into the primary signals that regulate the expression of nuclear-encoded translational activators. Nuclear-encoded proteins that regulate the translation of mitochondrial mRNAs have also been identified in yeast, however, the corresponding genes have not been isolated. Therefore, this research should provide important insight into the molecular mechanism(s) that signal the expression of nuclear-encoded translational activators, information which will be applicable to those systems utilizing nuclear/organelle cross-communication.